I can't really find much documentation on clustering openKM although some folks hint that they might be doing it already using standard java methodology. Is it possible today? Where exactly is clustering sitting in the roadmap. I know from other enterprise-open-source projects that companies like to hold on to the clustering features as pay-only. Will this be the case here?

We're working on clustering configuration now, testing several database engines etc... Normally as you said is not a comunity feature, this decision has not been taken at this moment. What I can inform at this moment is that some engines are no cost and others are propietary. Anyway for this kind of architectures is not good idea to do not have any kind of support. You can not get cluster with simple java, is more sophisticated than these. Here are several tomcats, several apache in front, a database in several severs, sometimes caches etc...

I do not know from where you get the idea, clustering is only activating activemq-5.13.2, it's something more complex than it. There are involved hardware and software configuration ( HA proxies, gluster fs, lucene configuration and optionaly database cluster )